In this specification we are particularly concerned with electronic paper displays, that is displays such as electrophoretic and electrowetting displays in which an electric field controls the appearance of a pixel, for example whether the pixel is black or white. Such displays are typically reflective, and thus easily visible in bright ambient light conditions and have a paper-like appearance as well as low power consumption. In electrophoretic displays typically small particles are dispersed in oil and the position and/or orientation of the particles is controlled by the applied voltage/electric field; in electrowetting displays the applied voltage/electric field typically controls the shape of a confined water/oil interface. Thus such electronic paper type displays may be characterised by use of a voltage/electric field to mechanically move some form of pigment or colouration which may be solid and/or liquid.
Current electronic paper displays provide some special problems compared with other technologies—they typically have a relatively low contrast ratio and a relatively limited number of different colours which can be displayed. We have previously described, in UK patent application GB1209301.9 filed on 23 May 2012 entitled “Electronic Display” (and in PCT/GB2013/051346) some techniques which may be employed to improve the apparent resolution of colour content presented on electronic paper displays. However other problems remain. In particular when, say, a chequer board of individual pixels is driven to an electronic paper display the result is not simply black and which and there is a region between pixels in some intermediate state between black and white. The width of this region is governed by the display design but tends to remain approximately constant regardless of display resolution. It arises from “fringing fields” and other electrically related phenomena. The result is that the smaller the pixels become, the less of the display is pure black and pure white.
In a typical colour electronic paper display the pixels are provided with coloured filter elements in a repeating pattern across the display so that individual pixels of the display show different colours. In order to achieve good colour performance it is important that as much as possible of the area of a pixel is at the intended grey level. However when representing a pure colour (ie the colour of one of the filter elements) adjacent pixels on the display to that of the pure colour are of opposite polarity to the activated pixels. For example if, say, displaying red on an electrophoretic display each “on” (white or reflective) red display pixel is surrounded by other display pixels that are “off” (black or non-reflective). If the display pixels are very small then none of the red pixels will ever reach full brightness and thus the colour will appear dim/unsaturated.
Thus it can be appreciated that for good colour reproduction larger display pixels are better, but in general the trend is towards ever higher pixel counts, increased resolution, and smaller pixels. We will describe techniques which address these difficulties.